In various kinds of combustion furnaces (combustion devices) using petroleum fuels such as heavy oils, “ORIMULSION”, etc., for example, boilers of thermal power plants and garbage incinerators, ammonia is added into combustion gases for preventing corrosion of the combustion furnaces by sulfate gases (SO3) contained in the combustion gases. In some cases, magnesium oxide may also be used instead of ammonia.
Meanwhile, “ORIMULSION” is a registered trademark owned by BITOR CO., LTD., Venezuela, for emulsion fuels prepared by dispersing natural asphalt produced at the basin of Orinoco River (Venezuela, South America) in water using a surfactant.
Accordingly, the combustion ashes collected and recovered by an electrostatic precipitator disposed at the downstream of a fire flue, contain ammonium sulfate or the like in addition to ashes composed mainly of unburned carbon and heavy metals (Ni, V, Mg, etc.). In particular, the ORIMULSION's combustion ashes have a less carbon content and high contents of ammonium sulfate (or magnesium sulfate) and vanadium. For example, the following components as shown in Table 1 below (based on dry solids) are contained in the combustion ashes recovered from a boiler using “ORIMULSION”.
TABLE 1Com-ponentsCNH4SO4VNiMgwt. %1 to 512 to 2050 to 654 to 90.5 to 20.1 to 2
As a wet-processing method of the combustion ashes, there have been proposed many techniques called a “wet-process” which are capable of not only recovering valuable components such as vanadium, but also taking suitable countermeasures against public pollution using a closed system. More specifically, there are known such wet-processes, for example, as described in Japanese Patent Application Laid-Open (KOKAI) Nos. 60-19086(1985) and 60-46930(1985) and Japanese Patent Publication (KOKOKU) Nos. 4-61709(1992) and 5-13718(1993) as already filed by the present applicant.
For example, the wet-process for combustion ashes of petroleum fuels as described in Japanese Patent Publication (KOKAI) No. 5-13718(1993) comprises (i) a first step of mixing combustion ashes with water by optionally adding thereto sulfuric acid for controlling the pH value to not more than 3, thereby preparing a slurry of the combustion ashes; (ii) a second step of separating solids (unburned carbon, etc.) from the slurry; (iii) a third step of heating the remaining liquid portion to a temperature of not less than 70° C. and oxidizing metals while feeding ammonia and an oxidant thereto for adjusting the pH value to 7 to 9; (iv) a fourth step of separating precipitated solids (iron sludge); (v) a fifth step of cooling the obtained liquid portion to a temperature of not more than 40° C., thereby precipitating vanadium compounds (ammonium metavanadate); (vi) a sixth step of separating the precipitated vanadium compounds; (vii) a seventh a step of adding calcium hydroxide or calcium oxide to the obtained liquid portion to precipitate gypsum and hydroxides of metals (nickel and magnesium), and liberating free ammonia simultaneously; (viii) an eighth step of stripping and recovering ammonia from the free ammonia; and (ix) a ninth step of separating the obtained gypsum.
However, the above-described large-scale processes are uneconomical and disadvantageous in such a case where the amount of combustion ashes to be treated is relatively small or where the combustion ashes have a less carbon content and a high vanadium content.
The present invention has been achieved in view of the above problems. An object of the present invention is to provide an industrially useful and simple process capable of readily recovering vanadium from combustion ashes which is suitably used in such a case where the amount of combustion ashes to be treated is relatively small or where the combustion ashes have a less carbon content and a high vanadium content.